1. Field of the Invention
The present invention relates to a method using ultrasound for non-destructive examination of a test body, whereby ultrasonic waves are coupled into the test body with one or a multiplicity of ultrasonic transducers and the ultrasonic waves reflected inside the test body are received by a multiplicity of ultrasonic transducers and converted into ultrasonic signals, which form the basis of the non-destructive examination.
2. Description of the Prior Art
The method of using ultrasound for non-destructive examination of a test body, for example for the purpose of examining material for flaws, such as cracks, occlusions or other inhomogeneities, comprises coupling ultrasonic waves into the test body, detection of the ultrasonic waves transmitted or reflected through the test body, deflected, scattered and/or broken in the test body as well as the evaluation of the ultrasonic waves converted into ultrasonic signals.
The preceding state-of-the-art method of examination permits determining and evaluating the ultrasonic transmission and ultrasonic reflection properties of a test body. In this method, which originates from medical technology (ultrasonic diagnostics), imperfections inside the test body, such as cracks, foreign occlusions or boundaries in the material are imaged by means of corresponding evaluation of the received ultrasonic signals as regions with altered reflection properties. Position, shape and size of the imperfections can be represented three-dimensionally in a spatially high-resolution manner.
It is obvious that the fields of application of this method are substantial and diverse, for example, the application of the method for examining and detecting the homogeneity or solidity properties of structural components (concrete walls, ceiling elements or wall elements, etc.) or for examining for cracks, for instance in railroad car wheels or aircraft parts.
Suited in an advantageous manner for coupling in, receiving and detecting ultrasonic waves are piezoelectric ultrasonic transducers which are able to convert electrical energy into elastic mechanical energy and inversely.
Piezoelectric ultrasonic transducers are distinguished, in particular, by their linear behavior during conversion of elastic mechanical energy into electrical energy and inversely. Moreover, depending on size and shape, piezoelectric ultrasonic transducers have an aperture, that is a specific emission characteristic which determines the spatial coupling-in behavior of the ultrasonic waves inside the test body. If a multiplicity of single ultrasonic transducers are employed, the coupling-in area of the single ultrasonic transducers can be assembled directly adjacent to each other on the test body in such a manner that the result is an overall aperture defined cumulatively by the apertures of the single ultrasonic transducers. In order to couple in, for example, ultrasonic waves with largely the same amplitudes in a half-space inside the test body volume, it is necessary to select ultrasonic transducers with an emission characteristic which is ball-shaped to the extent possible. If, however, the ultrasonic transmission of ultrasonic waves into a test body at as small as possible propagation angle is desired, it is necessary to select an ultrasonic transducer with as great as possible a “directional characteristic”.
In many applications using ultrasound for non-destructive material examination, a multiplicity of ultrasonic transducers are employed which are assembled in a so-called ultrasonic probe, or transducer for easier handling. Basically, it is necessary to differentiate between two types of transducers. If the transducer couples an ultrasonic wave package into the test body and the ultrasonic waves reflected inside the test body are received again, they are called impulse-echo probes. On the other hand, probes with separate ultrasonic transducers for coupling in the sound waves and receiving them again are referred to as transmission, and reception, probes.
In all prior art ultrasonic probes, the single ultrasonic transducers are each connected to a control device which is provided with separate control electronics for each ultrasonic transducer, that is each ultrasonic transducer has its own electrical control channel, in such a manner that single ultrasonic transducers can be activated separately and, for example, serve as an ultrasonic transmitter or an ultrasonic receiver. In particular, such type separate activation permits operating individual ultrasonic transducers, each with a different phase position and amplitude.
In order to conduct a measurement of the ultrasonic transmission capacity of a probe, the control device activates at least one of the ultrasonic transducers and usually a plurality of ultrasonic transducers, for a limited, brief interval to couple ultrasonic waves into the probe. The generated, coupled-in ultrasonic waves are, for example, reflected at imperfections inside the test body and are reflected as ultrasonic waves which return to the ultrasonic transducers. The ultrasonic transducers now operate as receivers of the ultrasonic waves which are converted into ultrasonic signals and are transmitted to the control device for evaluation. The interval between transmission and reception of the ultrasonic signals is usually referred to as a measurement period. For better signal detection and evaluation, a multiplicity of such measurement periods are conducted consecutively in order to obtain a tolerable signal-to-noise ratio.
Many applications call for as finely spatially resolved as possible determining the ultrasonic transmission properties and reflection properties of a test body inside the test body volume. To do so, a multiplicity of measurement periods are conducted in which the ultrasonic waves coupled into the test body are focussed onto a narrowly defined volume region, which is referred to as a “voxel”.
As a result of focusing the elastic energy of the ultrasonic waves on a certain volume region inside the test body, the elastic energy reflected from this volume region in the form of reflected ultrasonic waves is significantly larger than if the ultrasonic coupling-in is not focussed. Focussing enhances measurement sensitivity.
The “phased-array” method is used to focus the ultrasonic waves in a certain volume region inside the test body. The ultrasonic transducers are arranged in an array on the surface of the test body and are activated in a phase-shifted manner to the transmission of the ultrasonic waves, that is time staggered to the ultrasonic transmission. By means of suited selection of the time staggering, constructive overlapping of the coupled-in ultrasonic waves according to Huygen's principle occurs in a certain volume region. In order to achieve as optimum as possible constructive overlapping of the ultrasonic waves in the volume region, the individual ultrasonic transducers operating as ultrasonic transmitters must be activated with identical signal forms.
Apart from focusing ultrasonic waves onto a certain volume region inside the test body, it is also possible, by selection of the phase shift, to preset a uniform coupling-in direction of the ultrasonic waves for the activation of the arrayed ultrasonic transducers. In this manner, it is possible to couple pivotable ultrasonic fields into the test body.
Signal evaluation in the prior art phased-array method occurs in such a manner that the individual reflected ultrasonic signals received in one measurement period are summed cumulatively considered by taking into consideration the phase shift employed at the beginning of the measurement period during ultrasonic coupling-in. In this manner, a cumulative signal is formed after each single measurement period. Looking at all the cumulative signals together allows drawing conclusions about the ultrasonic transmission properties and reflection properties in the entire test body volume physically accessible for the material examination method.
A drawback in using the phased-array method for non-destructive material examination of a test body, however, is the amount of time and technical equipment required to examine a test body as completely as possible, since the point is to obtain sufficiently reliable measurement signals from all the volume regions for complete signal evaluation. For example, in one measurement period or a multiplicity of single measurement periods, it is only possible to obtain information about the reflection properties in only one volume region of the test body. Examination of the entire test body volume requires a very great number of measurements, each with different phase activation, making the entire material examination very time consuming.
Another disadvantage is that a preset ultrasonic coupling-in angle determines the probe aperture so that the aperture cannot be optimally selected for all ultrasonic coupling-in angles which yields poorer measurement resolution.
A further drawback of the phased-array method is that a transmission channel and a reception channel has to be provided for each ultrasonic transducer with corresponding activation electronics, which has to be connected to the respective ultrasonic transducer via separate electrical connections. As the presently used ultrasonic probes usually comprise 16 or more ultrasonic transducers, connection between the probe and the control device usually requires a thick, inflexible and consequently unwieldy cable.